Method of in-situ formation of a stable reference electrode for in-tank plating bath analysis

ABSTRACT

A method of forming a reference electrode having a continuously stable reference voltage and particularly well-suited for use in an in-tank electrochemical sensor. The method utilizes an inert substrate and a counter electrode, both immersed in electroplating fluid. Current is passed between the inert substrate and the counter electrode to strip and subsequently replate the inert substrate, which then serves as the reference electrode. The steps of stripping and replating the reference electrode are periodically repeated to maintain the stability of the reference electrode voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of in-situ formation of areference electrode for in-tank use in plating bath analysis. Moreparticularly, the present invention relates to a method for repeatedlyregenerating a reference electrode to provide long-term referencevoltage stability without replacing the electrode. The stable referenceelectrode is of the type ideally required in in-tank sensors used toaccurately measure and analyze the electrochemical properties of platingbaths.

2. Description of Related Art

Plating bath analysis methods, such as the method disclosed in U.S. Pat.No. 4,631,116, and assigned to the present common assignee, typicallyuse electrochemical sensors containing sensing electrodes and areference electrode. The sensing electrodes usually include a workingelectrode and a counter electrode. All of the electrodes are in directcontact with the plating bath solution. The electrochemical propertiesof the plating bath solution are measured by applying ac and dc signalsto the solution via the sensing electrodes and measuring the resultantresponse signals. The reference electrode plays an important role in theproper functioning of the sensor, since it provides a reference voltageupon which the various ac and dc signals are applied and controlled atthe sensing electrode during measurement.

The reference voltage is a function of the particular type of platingbath, and reflects changes in the bath that are independent of theapplied signals. Basing response signal measurements on this referencevoltage permits accurate monitoring of a variety of importantelectrochemical properties. Absent the reference voltage, normalground-referenced voltage variations for a given plating bath willinterfere with the control and response signals, preventing robust andaccurate electrochemical analysis.

Commercially available standard reference electrodes are not suitablefor in-tank use and suffer from the following limitations: frequent andtime consuming maintenance, contamination of the plating bath, fouling,unsatisfactory stability, or structural characteristics not readilycompatible with in-tank sensor designs.

Alternatively, wire reference electrodes with material matching theplating bath have been suggested for use. U.S. Pat. No. 4,812,210describes various reference electrodes of this type. The wire may alsobe formed from a base metal substrate suitably plated for a particularapplication. Usually, the base metal is the same metal as the platingapplication or a less expensive material.

The reference electrodes described above suffer from a number of defectswhich render them inadequate for in-tank applications. The referenceelectrodes require costly and time-consuming maintenance, since theyfrequently wear out and must be replaced. Moreover, the plating on aplated reference electrode deteriorates over time, and can eventuallyflake off and contaminate the bath. The flakes can also accumulate in anelectrochemical sensor and thereby interfere with the operation of theworking and counter electrodes.

Furthermore, the currently utilized reference electrodes exhibit avoltage instability which is unacceptable in the many plating bathapplications which require continuous operation for periods of severalweeks or more. For example, tests performed on a solid copper wirereference electrode showed that the electrode voltage became unstable inless than 24 hours. This type of voltage instability significantlydegrades the accuracy and efficiency of voltammetric analysis techniquessuch as those described in U.S. Pat. No. 4,631,116. A user requiringcontinuous analysis of the plating bath must either accept continuallydegrading measurement accuracy or interrupt production to manuallyreplace the reference electrodes several times a day.

As is apparent from the above, there presently is a need for a stablereference electrode for in-tank use which does not require replacementor generate contaminants within the plating bath or sensor. Further, thereference electrode should provide a stable voltage reference during theelectrochemical analysis and over long periods of time. The referenceelectrode should provide these features and also be compatible with mostin-tank electrochemical sensors, plating baths and the measurementmethods and equipment associated therewith.

SUMMARY OF THE INVENTION

In accordance with the present invention a method is provided forforming a stable reference electrode. The reference electrode isparticularly well-suited for use in an in-tank electrochemical sensor ofthe type used to monitor plating baths by ac and dc voltammetry.

The method involves providing an inert reference electrode substrate anda counter electrode, both of which are immersed in electroplating fluid.A sufficient current is passed between the inert substrate and counterelectrode to remove previous layers of metal plating and othercontaminants from the inert substrate and otherwise prepare thesubstrate for plating. An electroplating current is then passed betweenthe inert substrate and the counter electrode in order to electrodeposita fresh layer of metal on the cleaned and prepared inert substrate. Themethod thus quickly and efficiently forms the reference electrode bystripping and replating its inert substrate.

As a feature of the present invention, the reference electrode so formedis regenerated rather than replaced, essentially eliminating themaintenance time and expense associated with reference electrodescurrently used. The inert reference electrode substrate is formed of amaterial able to withstand wear from repeated stripping and replating,and therefore rarely requires replacement.

In accordance with the present invention, the steps of completelystripping and then replating a new deposit each time prevents excessivematerial build-up and, thereby, eliminates problems associated withflaking of the reference electrode. The method of the present inventionthus prevents the contamination of the plating bath and interferencewith sensor measurements associated with reference electrode flaking.

As an additional feature of the present invention, the referenceelectrodes so formed are readily adaptable for use within many differentin-tank electrochemical sensors and measurement systems. The method offormation of the present invention thus eliminates the need for customreference electrode design and can provide a standard referenceelectrode which is structurally compatible with most in-tank sensors.

As another feature of the present invention, the method may be used witha variety of different plating baths. An accurate and stable referenceelectrode can be formed as long as there is sufficient metallic contentin the bath.

In accordance with the present invention the reference electrodeformation method may be performed using the same equipment andinstrumentation typically used with voltammetric plating bath analysismethods such as those disclosed in U.S. Pat. No. 4,631,116. The methodof the present invention is thus easily integrated into known platingbath analysis techniques to further improve the flexibility andefficiency of an overall electrochemical monitoring system.

As a further feature of the present invention, the formation of thereference electrode is accomplished very rapidly so as to not undulyinterfere with plating bath analysis. The newly deposited layer of metalplating exhibits a rapidly equilibrated metal-solution interface and itis thus possible to continue making accurate electrochemicalmeasurements almost immediately after the reference electrode isregenerated.

The above-discussed features and attendant advantages of the presentinvention will become better understood by reference to the followingdetailed description of the preferred embodiments and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an exemplary in-tank electrochemical sensorincorporating a preferred embodiment of a reference electrode formed inaccordance with the method of the present invention.

FIG. 2 is a side sectional view of the exemplary electrochemical sensorof FIG. 1 showing the location of the reference electrode within thesensor.

FIG. 3 is a detailed side sectional view of the reference electrodeinstalled within the sensor of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary electrochemical analysis methods and equipment, to which themethod of forming a stable reference electrode of the present inventionis applied, are described in U.S. Pat. No. 4,631,116, which has beenpreviously discussed. The contents of this patent are hereby expresslyincorporated by reference. The application of plating bath analysismethods such as those described in U.S. Pat. No. 4,631,116, for in-tankuse are made possible through the in-situ formation of a referenceelectrode which yields stable voltages during the electrochemicalmeasurements and over substantial periods of time.

The method of the present invention uses an inert material, preferablyformed of platinum or gold as the substrate of the reference electrode.A counter electrode also made of inert material is required to provide,in conjunction with the inert substrate, a path for current to beapplied for stripping and plating the inert substrate. The substrate andcounter electrodes are both submerged in electroplating solution. Theelectroplating solution comprises metals which are capable of beingelectroplated, including, but not limited to, copper, iron, nickel,chromium, zinc, gold, silver, lead, platinum, cadmium, palladium,rhodium, indium, cobalt, tin and mixtures therefor.

First, any residual plated material or other contaminants on the inertsubstrate are stripped away via the application of sufficient currentbetween the substrate and counter electrode. This current is preferablya constant DC current with a current density of about 100 to 1000milliamperes per square centimeter, but other current waveforms may besuitable as well. Among the contaminants on the inert substrate whichare removed in this step are plating from previous electrode formation,adsorbed organics, and adsorbed inorganics. The application of a highlevel of constant current insures complete removal of all contaminants.The current is applied until substantially all of the contaminants areremoved from the substrate. The time required for this will varydepending upon a variety of parameters. In general, application of thecurrent for between about 10 and 60 seconds is sufficient. The currentis applied from an external source, which may be the same source as thatused for the sensor which is described below.

After the substrate has been cleaned and prepared, current is againapplied to electrodeposit a layer of metal plating on the substrate. Apreferred current waveform is a pulsed waveform with a current densityof about 50 to 300 milliamperes per square centimeter, a period of about1 to 10 seconds and a pulse duration of about 100 to 1000 milliseconds.It should be noted that any suitable electroplating current waveform canbe used in this step, including constant current waveforms. The metalplating can be copper or any other metal suitable for the givenapplication.

By way of example, the invention is described for an acid copper platingbath application. First, a constant anodic current density of 750milliamperes per square centimeter is applied to an inert platinumsubstrate for a period of 20 seconds. This current serves to strip offany copper from the previous formation procedure, oxidize any adsorbedorganic material on the platinum substrate, and generally prepare thesubstrate for the next step of electrodeposition.

Next, a pulsed current waveform with a peak current density of about 250milliamperes per square centimeter, a period of about 1 second, and apulse duration of about 100 milliseconds is used to electrodeposit alayer of copper on the platinum substrate. The resultant referenceelectrode reaches about 98 percent of its final steady state voltagewithin about 10 to 20 seconds. Although other electroplating waveformscould be used for acid copper plating baths, the pulsed current waveformwas found to yield reference electrodes with better voltage stability.

A summary of data obtained for seven reference electrode formationtrials is presented in Table 1. The reference electrode so formedmaintains a very stable voltage with a drift of less than or equal to0.4 mv/min. The reformed electrodes provide open circuit voltages thatvary about ±2 mv. Therefore, numerous highly accurate electrochemicalmeasurements can be performed before reforming the reference electrode.

                  TABLE 1.                                                        ______________________________________                                        Summary of reference                                                          electrode formation trials                                                               Electrode Voltage                                                                           Voltage Drift                                        Trial      (mv vs. SCE)  (mv/min)                                             ______________________________________                                        1          60            0.38                                                 2          63            0.36                                                 3          63            0.4                                                  4          58.6          0.38                                                 5          59.7          0.36                                                 6          59.2          0.36                                                 7          59.5          0.32                                                 ______________________________________                                    

FIGS. 1 and 2 show a side view of an exemplary in-tank electrochemicalsensor 10 suitable for use with the method of the present invention andthe plating bath analysis techniques described in U.S. Pat. No.4,631,116. The sensor 10 is immersed in a plating tank 24 filled withelectroplating liquid 25. The liquid 25 flows through the sensor 10 viaa pump 23 and tubing 22. The pump 23 draws the liquid 25 into the sensor10 through inlet tubes 28, 29 and inlets 30, 31. The liquid then passesthrough the sensor 10 and pump 23 and back into the tank 24. Counterelectrode 40 and working electrode 41 serve as sensing electrodes forelectrochemical measurement. In traveling through the sensor 10, theliquid passes sensing electrodes 40, 41 and finally a referenceelectrode 39. The sensing electrodes 40, 41 are powered and monitoredvia wires 50 which enter the sensor through bushings 51 and areprotected by shielding 48. All sensing electrode measurements are takenrelative to the reference voltage supplied by the reference electrode39.

The exemplary reference electrode 39 is connected to an external currentgenerating and control apparatus via wires 35, 36 which enter into thesensor 10 via leak-proof bushing 26. Electrode wires 35, 36, 50 aresupported and protected by shielding tube 21 as they exit the platingtank 24. The reference electrode 39 is preferably located within theinterior 37 of the fluid exit chamber 27 of sensor 10 above sensingelectrodes 40, 41. However, reference electrode 39 can be locatedelsewhere in the sensor 10.

FIG. 3 shows exemplary reference electrode 39 in greater detail. In thepreferred embodiment shown, the reference electrode 39 is formed on aninert platinum substrate extending upward from a securing base 38 andsurrounded by two counter electrodes 45, 46. An appropriate current ispassed between the reference electrode 39 and counter electrodes 45, 46via wires 35, 36 entering the sensor 10 through leak-proof bushing 26.One of the wires 35, 36 is in electrical contact with the inertsubstrate while the other is in electrical contact with the counterelectrodes 45, 46. Both wires 35, 36 are conveyed to their respectiveelectrodes under shielding 34. The counter electrodes 45, 46 arepreferably in close proximity to the reference electrode. All electrodesare immersed within the electroplating liquid 25 which passes throughthe sensor 10 during electrochemical analysis.

The shape and location of the reference and counter electrodes shown inFIG. 3 are exemplary only and many alternative shapes, locations, andarrangements of these elements are possible. For example, electrodes 45,46 could serve as inert substrates upon which reference electrodes areformed and electrode 39 could then serve as a counter electrode.

Although the above description has been limited to reference electrodesfor use within in-tank electrochemical sensors used for plating bathanalysis, this is by way of illustration and not limitation. Forexample, the method described herein could also be useful in many otherplated electrode applications. It will be understood by those skilled inthe art that many alternate implementations of this method are possiblewithout deviating from the scope of the invention, which is limited onlyby the appended claims.

I claim:
 1. A method for forming a reference electrode having a stablereference voltage and adapted for use in an in-tank electrochemicalsensor, said method comprising the steps of:providing at least one inertsubstrate; providing at least one counter electrode; providing a liquidcontaining a metal which is electroplated onto said inert substrate;passing a level of current between said inert substrate and said counterelectrode for a time to thereby remove contaminants which are present onsaid inert substrate to provide a stripped inert substrate wherein saidcurrent applied to remove contaminants from said inert substrate is acurrent waveform with a current density of about 100 to 1000milliamperes per square centimeter; and passing a level of currentbetween said stripped inert substrate and said counter electrode for atime to electrodeposit a layer of said metal from said liquid onto saidstripped inert substrate to form said reference electrode.
 2. The methodof claim 1 wherein said steps of passing said level of current to removecontaminants and passing said level of current to electrodeposit a layerof metal are repeated periodically to thereby provide a referenceelectrode with a consistently stable reference voltage duringelectrochemical measurements and over long periods of time.
 3. Themethod of claim 1 wherein said liquid is an electroplating liquid whichcontains said metal.
 4. The method of claim 1 wherein said inertsubstrate is formed from a metal selected from the group consisting ofplatinum and gold.
 5. The method of claim 1 wherein said current appliedto remove contaminants from said inert substrate is a constant currentwaveform applied continuously for a period of about 10 to 60 seconds. 6.The method of claim 1 wherein said current applied to electrodeposit alayer of metal on said substrate is a current waveform with a currentdensity of about 5 to 300 milliamperes per square centimeter.
 7. Themethod of claim 1 wherein said current applied to electrodeposit a layerof metal on said inert substrate is a periodic current waveform with aperiod of about 1 to 10 seconds.
 8. The method of claim 1 wherein saidcurrent applied to electrodeposit a layer of metal on said inertsubstrate is a pulsed current waveform with a pulse duration of about100 to 1000 milliseconds.
 9. The method of claim 1 wherein saidreference voltage reaches about 98 percent of its steady-state voltagevalue about 10 to 20 seconds after the formation of said referenceelectrode.
 10. The method of claim 3 wherein said electroplating liquidincludes a metal selected from the group consisting of copper, iron,nickel, chromium, zinc, tin, gold, silver, lead, platinum, cadmium,palladium, rhodium, indium, cobalt and mixtures thereof.